Leaking tank leaking at two of the bolts that connect the brass top to the black tank

The tank is leaking and in need of replacement or a connection at the top is leaking

Find the cold water supply pipe to the tank. Most are labeled, but if unsure - turn on the closest hot water faucet and feel the temperature of the pipes connected to the water tank. Follow the coldest pipe back (moving away from the tank) until you reach the first shut off valve on that pipe and shut it off completely.
No more cold water will fill the tank and cause water to spray / leak from the pipe.
There is no need to shut off the gas really, as it is controlled by the temperature sensor on the tank - but if you insist, you should find either a flexible pipe - or a black iron pipe that enters a controller. Most have a dial marked "OFF" "ON" & "PILOT". Simply twist the dial from ON to OFF and the gas will be shut off.
Good luck!

Copy following link for troubleshoot resources:
http://waterheatertimer.org/Leaking-water-heater.html

Replace rusted tank
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Prevent rusted tank.
If nipples are leaking, and they did not leak before, then tank has most likely rusted out and needs replacement.
Cause for rusted tank is depleted anode rod.
Inspect and replace anode each 1-3 years.
http://waterheatertimer.org/Replace-anode-rod.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Copy following link for general troubleshoot:
Then add a comment and let's get the problem solved.
http://waterheatertimer.org/Leaking-water-heater.html

FYI:
Anode rod is either connected to hot-water nipple, or installed in separate port located top of tank. Anode stops water heater tank from rusting.
Dip tube is a plastic tube that is connected to cold water nipple and extends to bottom of tank so cold water always enters tank bottom.
TP valve is brass safety valve that is screwed into a port located on top of tank, or located on side of tank, and will release water when pressure or temperature exceed rating of TP.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Are you installing nipple in top of tank-type water heater?
Did you use 6 wraps teflon tape?
Is the nipple IP iron pipe tapered thread? Or is it straight thread?

Why are you replacing nipple?
Was old nipple leaking?
If previously sealed pipe connection to water heater has started leaking, then it implies the tank is rusted, and water heater needs to be replaced.
http://waterheatertimer.org/Leaking-water-heater.html

Have you checked the anode rod.
http://waterheatertimer.org/Replace-anode-rod.html

Why are you using brass instead of pex-lined di-electric nipple, available from box store?
http://waterheatertimer.org/images/IMG_1879-di-electric-nipple-800.jpg

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

just turn off the gas valve for pilot light, turn off cold water to water tank. drain system. when refilling open cold water to fill tank, wait at least a 1/2 hour to 45 minutes then relight water heater. you should be fine!

First shut off power then remove acess cover and carefully move insulation out of the way so you can see around elements.Check to see if leak is coming from around element.If it is determine what type of element you have. Either screw in or bolted in. Both have thick rubber gaskets that can be replaced by draing water heater and removing element then install new gasket and reinstall element.
To drain heater shut off valve on incoming cold water supply line ,make sure electric power is off. Connect drain hose to drain valve and put other end to floor drain or sump basin.You can open faucet on hot side near by to prevent vacumn and drain. If you have screw in type disconnect wires,marking them for reattachment,then use large open end wrench or pipe wrench on hex section of element and turn to left to remove. If you have bolt in element spray bolts with wd-40 or similar and carefully loosen bolts then disconnect wires and remove element.install new gasket and reverse to reinstall. I would recommend that you replace elements at this time if you are sure this was the source of the leak.
If you don't think it's leaking around elements then check fitting on both water lines to water heater to be sure they are not leaking and seeping down thru insulation and leaking out of the bottom of heater. Next check around drain valve for leak and if it's leaking you can try tightening it a little or replace it with 3/4 x2 brass nipple and 3/4 inch female iron pipe boiler drain. Use teflon tape or pipe joint compound on all threaded connections.
After you have eliminated all other sources for leak then it is probably the tank itself. If it is less then six years old the tank should still be covered under warrenty. Most warrenties will give you a new heater but installation will not be covered unless you have extended service coverage.
I hope this helps you some. Thank you

You can try to crack it open and get a little piece of crud out of there, I'm sure that is all it is, Just get a tray or something to catch the water, It should stop it, If it gets worse, turn the water off to the heater, then open the valve and hope the bigger piece of crud comes out. It is about all you can do, other than drain the whole thing and take the valve apart and replace the rubber in it. Check it out, Hope this helps.

It sounds like two problems.
Problem 1) Clog inside faucet or clog inside pipes at faucet
Remove supply tubes leading to faucet and see if problem is inside pipes or inside faucet.
My bet is clog inside faucet.

If you have galvanized pipe that meets copper pipe, this can be trouble spot.
Article on clogged water pipes at Inspectapedia:
http://www.inspectapedia.com/water/Water_Pipe_Clog_Repair_Guide.htm

Article on clogged pipes:
http://www.waterheaterrescue.com/pages/WHRpages/English/Troubleshooting/Tanklets/backflushing.html

Problem 2) Water heater leak ... you say tank valve is leaking? Is that cold-water shut-off valve? Or tank drain valve? Or TP valve?

If cold water shut-off is leaking, then replace shut off.

If tank drain valve is leaking, then check if plastic valve is cracked. If drain valve is cracked, then replace with a brass valve from Home Depot. If plastic valve breaks off, then use hammer and screwdriver to gently chip out the broken plastic valve. New valve threads need teflon tape to seal pipe. If valve is just dripping, then put garden-hose cap over end of valve and screw down tight.

If TP valve is leaking, then replace with same temperature and pressure rating. New TP valve threads need teflon tape to seal pipe.

Problem 3) If pressure is low all over house:
Buy pressure gauge at hardware store that screws onto hose connection.

Test pressure on outdoor spigot.
Open faucet and check if pressure drops. This says if problem is inside pipes that enter house.

Test pressure on water heater drain valve.
This says if pressure problem is before -or- after water heater.

If problem is before the water heater, then shut-off valve located on cold water line is suspect.
If problem is after water heater, it could be in hot water outflow pipe on top of water heater, or a nearby elbow.
Remove hot water line leading from water heater and check pressure. This says if problem is where hot water leaves tank.
Read articles above about clogs inside water lines.

You need to check for a leak from one of the supply lines, if they aren't leaking, then the unit has developed from the tank. If that is the cause, you will have to replace the water heater.